The invention relates to a differential pressure pickup with a sensor element, a first and a second pressure measuring chamber, respectively adjacent to the sensor element, a first pressure receiving chamber, closed off by a first separating diaphragm, a second pressure receiving chamber, closed off by a second separating diaphragm, a first overload chamber, which is connected to the first pressure receiving chamber by a first line and which is connected to the first pressure measuring chamber by a second line, a second overload chamber, which is connected to the second pressure receiving chamber by a third line and which is connected to the second pressure measuring chamber by a fourth line, a liquid filling the first and second pressure receiving chambers, the first and second overload chambers, the first and second pressure measuring chambers and also the first, second, third and fourth lines, and an overload diaphragm, by which the first and second overload chambers are separated from each other.
Pressure pickups are usually able to be connectable by means of process connections, so that a first pressure acts on the first separating diaphragm and a second pressure acts on the second separating diaphragm. The pressures are transmitted into the pressure measuring chambers via the liquid and they bear against the sensor element. The sensor element, for example a piezoresistive differential pressure measuring cell, emits an output signal which is proportional to the difference between the first pressure and the second pressure. The output signal is available for further processing, evaluation and/or display.
Differential pressure pickups are used for measuring differential pressure as a process variable in many applications. Apart from the differential pressure as such as a measured variable, a differential pressure pickup can also be used to determine a filling level in a container on the basis of a hydrostatic pressure differential. Equally, a flow rate can be determined for example from the difference in pressure between two different locations of different cross section in a channel.
DE-A 42 44 25 7 describes a differential pressure pickup with a sensor element, a first and a second pressure measuring chamber, respectively adjacent to the sensor element, a first pressure receiving chamber, closed off by a first separating diaphragm, a second pressure receiving chamber, closed off by a second separating diaphragm, a first overload chamber, which is connected to the first pressure receiving chamber by a first line and which is connected to the first pressure measuring chamber by a second line, a second overload chamber, which is connected to the second pressure receiving chamber by a third line and which is connected to the second pressure measuring chamber by a fourth line, a liquid filling the first and second pressure receiving chambers, the first and second overload chambers, the first and second pressure measuring chambers and also the first, second, third and fourth lines, and an overload diaphragm, by which the first and second overload chambers are separated from each other.
In the case of the pressure pickup described, the overload diaphragm is in the form of a disk and is firmly restrained at its outer edge.
In the case of overloading, i.e. if a pressure which is greater than a permissible upper limit value for which the pressure measuring pickup is designed acts on one of the separating diaphragms, the separating diaphragm is pressed against its diaphragm bed. The volume of liquid displaced by the deflection of the separating diaphragm passes through one of the lines from the pressure inlet chamber into the assigned overload chamber and leads to a deflection of the overload diaphragm from its zero position. In the volume additionally available on one side as a result of the deflection of the overload diaphragm, at least part of the displaced volume of liquid is accommodated for the duration of the overload. The pressure which acts on the sensor element in the case of overloading is limited in this way and the sensor element is protected against being overloaded.
In the case of overloading, very great forces can act on the overload diaphragm very quickly. Very great stresses occur in particular where the overload diaphragm is restrained at its outer edge, and can result in plastic deformations, i.e. deformations which remain even after the overloading has diminished. In a corresponding way, the overload diaphragm does not return to its original zero position and both overload chambers have a volume which has changed in comparison with the original state. The difference between the volumes of an overload chamber before and after the occurrence of the case of overloading is referred to below as the hysteresis volume. The hysteresis volume has a direct effect on the pressure distribution in the pressure pickup and consequently falsifies the measurement result.
It is an object of the invention to specify a differential pressure pickup of the type stated above which has a low hysteresis volume.
For this purpose, the invention comprises a differential pressure pickup of the type stated at the beginning in which the overload diaphragm has a closed outer edge and a closed inner edge and is firmly restrained along its outer edge and its inner edge.
According to one embodiment of the invention, the overload diaphragm is in the form of an annular disk.
According to a further embodiment, each overload chamber is bounded by the overload diaphragm and an essentially concave wall lying opposite the latter.
According to one embodiment of the invention, one of the lines is led through the center of the overload diaphragm.
One advantage of such a differential pressure pickup is that it has a very low hysteresis volume in comparison with pressure pickups with overload diaphragms in the form of a disk.
An infinitesimal annular disk segment of the width xcex94r deflected by a distance y from its zero position encloses under it a volume xcex94V=y(r)*(2xcfx80r xcex94r), which is proportional to its radius r. Plastic deformations occurring specifically at the outer edge of an overload diaphragm therefore account for a great proportion of the hysteresis volume.
In the case of overloading, the forces acting on an overload diaphragm designed according to the invention are distributed proportionately between its inner restraint and outer restraint. Accordingly, stresses occurring at the outer restraint, and consequently also the resultant plastic deformations, are less than in the case of a separating diaphragm without an inner restraint.